As one of devices used for detecting a rotation angle of a machine shaft, a magnetic encoder has been known. The magnetic encoder includes a magnetic drum having a ring-shaped magnetic material (a magnet) in which a magnetic pole array is magnetized on the outer circumference, and a magnetic resistance element (MR element) closely arranged on the outer circumference of the magnetic drum. The magnetic drum is mounted on the shaft of the machine to be detected and rotates together with the shaft, and by detecting a change of a magnetic field associated with the rotation of the magnetic drum by the MR element, a rotation angle of the shaft to be detected is detected.
The magnetic drum is configured by a base material made of a magnetic body and a ring-shaped magnet fixed to the outer circumference of the base material. A centrifugal force is acting all the time on the rotating magnetic drum, and the centrifugal force acts as a force that takes the magnet off from the outer periphery of the base material. The centrifugal force of the magnetic drum rotating at a high speed can be a factor causing a breakage of the magnet. Therefore, the magnet and the base material need to be fixed to each other firmly in order to ensure the reliability of the magnetic encoder.
For fixation of the base material and the magnet, an adhesive is used. However, because an application work of the adhesive is performed manually, variability is likely to occur in the process. In a portion where insufficient filling of the adhesive or entrainment of bubbles has occurred, bonding (fixation) strength decreases, thereby the portion can be a factor causing breakage of the magnet.
Furthermore, uneven thickness of the adhesive becomes a factor of generating vibration during rotation of the magnetic drum.
When the drum is to be produced stably and manually while preventing these problems, an increase in production costs is unavoidable.
Patent Literature 1 discloses a technique in which a groove is provided at an outer peripheral end edge of a disk-shaped wheel as a base material, and an annular magnet is injection molded in the groove.